If you fly a lot on United, their mileage Club card is great. Gives you access to their lounges and bonus miles for booking. You can also get the first year fee waived if you go to a Chase branch to open the card.
A little-known fact: if you want to open a new credit card, do it at your local bank branch. At least in the US, at the branch they have to give you the best offer currently available on the market for the card.